|Jmol-3D images||Image 1|
|Molar mass||96.198 g/mol|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)|
The S3 molecule or trisulfur or sulfur trimer or thiozone or triatomic sulfur is an allotrope of sulfur. It occurs as a mixture in liquid and gaseous sulfur and also at cryogenic temperatures as a solid. Under standard conditions it is unstable and self reacts to solid sulfur cyclooctasulfur. The molecule shape is similar to ozone. S3 is found in sulfur vapour, comprising 10% of vapour species at 713 K (440 °C) and 1333 Pa (10 mmHg). It is cherry red in colour, with a bent structure, similar to ozone, O3.
The molecule has a distance between sulfur atoms of 191.7(1) pm (1.917(1) Å) and angle at the central atom of 117.36(6)°. However, cyclic S3, where the sulfur atoms are arranged in an equilateral triangle with three bonds (similar to cyclic ozone), should in theory be lower in energy than the bent structure actually observed.
The name Thiozone was invented by Hugo Erdmann in 1908 who hypothesized that S3 made up a large proportion of liquid sulfur. However its existence was unproven until the experiments of J. Berkowitz in 1964. Using mass spectrometry, he showed that sulfur vapour contains the S3 molecule. Above 1200°C S3 is the second most common molecule after S2 in sulfur gas. In liquid sulfur the molecule is not common until the temperature is high, such as 500°. However small molecules like this contribute to most of the reactivity of liquid sulfur. S3 has an absorption peak of 425 nm with a tail extending into blue light.
S3 can also be made by photolysis of S3Cl2 embedded in a glass or matrix of solid noble gas.
Natural occurrence 
It occurs naturally on Io in volcanic emissions. Also S3 is likely to appear in the atmosphere of Venus at 20 to 30 km height, where it is in thermal equilibrium with S2 and S4. The reddish colour of Venus atmosphere at lower levels is likely to be due to S3.
Formation of compounds with a defined number of sulfur atoms is possible:
S3 + S2O → S5O (ring)
Radical anion 
S3 can form the radical anion S3−, which has an intense blue colour. The ion is also called thiozonide, by analogy with the ozonide anion, O3−. The gemstone lapis lazuli and the mineral lazurite (from which the pigment ultramarine is derived) contain S3−. International Klein Blue, developed by Yves Klein, also contains the S3− radical anion. This is valence isoelectronic with the ozonide ion. The spectrum of the colour shows a strong absorption band at 610–620 nm or 2.07 ev. The blue colour is due to the C2A2 transition to the X2B1 electronic state in the ion. The Raman frequency is 523 cm−1 and another infrared absorption is at 580 cm−1.
The S3- ion has been shown to be stable in aqueous solution under pressure of 0.5 GPa, and is expected to occur naturally at depth in the earth's crust where subduction or high pressure metamorphism occurs. This ion is probably important in movement of copper and gold in hydrothermal fluids.
The S3- radical anion was also made by reducing sulfur gas with Zn2+ in a matrix. The material is strongly blue coloured when dry and changes colour to green and yellow in the presence of trace amounts of water. Another way to make it is with polysulfide dissolved in hexamethylphosphoramide where it gives a blue colour.
Raman spectroscopy can be used to identify S3-, and it can be used non-destructively in paintings. The bands are 549 cm−1 for symmetric stretch, 585 cm−1 for asymmetric stretch, and 259 cm−1 for bending. Natural materials can also contain S2- which has an optical absorption at 390 nm and Raman band at 590 cm−1.
Trisulfide ion 
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